However, authorities are concerned that tensions between Shiite militias and Mosul’s Sunni Muslim population may lead to revenge killings in the heat of battle.

Mosul is the largest ISIL held city in Iraq. With a pre-war population of around 2 million people, it is far bigger than any other cities that have been recaptured from the militant group so far.

The United Nations said last week that it was bracing for the world’s biggest and most complex humanitarian effort. The offensive could leave up to 1 million homeless and may see civilians used as human shields or even gassed.